Uneven ventilation of parallel lung units is associated with phase differences between the local alveolar pressures in the individual compartments. These phase differences can be sensed by whole body plethysmography when the subject is breathing normally. It appears as phase lag of total alveolar gas compression with respect to gas flow at the mouth. In preliminary experiments we studied these phase differences in a theoretical model and in a limited number of subjects including normals, patients with known uneven ventilation and in young smokers. The results indicate that various degrees of uneven ventilation can be sensed by this method; the results in young smokers indicate that the method is very sensitive. I propose to study body plethysmography in more detail as a means of getting a better insight into the dynamics of uneven ventilation. Indeed, plethysmography is unique in being the only method which detects temporal phase differences between flow and pressures in parallel compartments during normal breathing. A number of other tests (closing volumes, frequency dependent compliance, flow volume curves) will be done for comparison. The subjects will be selected from a group of about 200-300 smokers. The technique will also be used in a later stage of the project to study basic features of uneven ventilation during other respiratory maneuvers such as coughs, forced expirations, and the procedure used for the determination of closing volumes.